The issue appears to have something to do with DNS hosts — in
particular, Dyn, one of the biggest DNS companies.

Domain Name Servers are a core part of the internet's backbone.
They translate what you type into your browser
—www.businessinsider.com, for example — into IP addresses that
computers can understand.

Dyn said on Friday that it suffered a DDoS attack, or a
distributed denial of service. That basically means hackers are
overwhelming Dyn's servers with useless data and repeated load
requests, preventing useful data — the Twitter IP address, for
example — from getting through.

"The purpose of this attack is to overload the service in any way
possible and make it stop working or be unreachable. In this case
it was not Twitter or Github that got overloaded, those services
work totally fine, but a service allowing you to reach them got
overloaded," Adam Surak, site reliability engineer at Algolia.com told Business
Insider.

Who is responsible?

No group has taken credit for the DDoS attack yet, and Dyn says
no attacker has contacted it.

The attack does not seem to be state-sponsored or directed, a
senior US intelligence official
told NBC News.

Dyn says that the attacks are "well planned and executed, coming
from tens of millions of IP addresses at the same time." One of
the sources of the attack is internet-connected products
like printers, DVRs, and appliances, often called the "internet
of things."

WikiLeaks says the attack is being done in support of its founder
Julian Assange.
Brian Krebs, a writer who was the first person to be hit with
a internet of things DDoS, believes that criminals are extorting
internet infrastructure companies and threatening them with DDoS.

Timeline of the attacks:

"Starting at 11:10 UTC on October 21th-Friday 2016 we began
monitoring and mitigating a DDoS attack against our Dyn Managed
DNS infrastructure. Some customers may experience increased DNS
query latency and delayed zone propagation during this time.
Updates will be posted as information becomes available."

8:45 a.m. ET:

"This attack is mainly impacting US East and is impacting Managed
DNS customer in this region. Our Engineers are continuing to work
on mitigating this issue."

9:36 a.m. ET:

"Services have been restored to normal as of 13:20
UTC."

As of 12:06 p.m. ET, the attack had returned:

"As of 15:52 UTC, we have begun monitoring and mitigating a
DDoS attack against our Dyn Managed DNS infrastructure. Our
Engineers are continuing to work on mitigating this
issue."

12:48 p.m. ET:

"This DDoS attack may also be impacting Dyn Managed DNS advanced
services with possible delays in monitoring. Our Engineers are
continuing to work on mitigating this issue."

1:53 p.m. ET:

"Our engineers continue to investigate and mitigate several
attacks aimed against the Dyn Managed DNS infrastructure."

2:23 p.m. ET:

"Dyn Managed DNS advanced service monitoring is currently
experiencing issues. Customers may notice incorrect probe alerts
on their advanced DNS services. Our engineers continue to monitor
and investigate the issue."

2:52 p.m. ET:

"At this time, the advanced service monitoring issue has
been resolved. Our engineers are still investigating and
mitigating the attacks on our infrastructure."

3:44 p.m. ET:

"Our engineers are continuing to investigate and mitigate several
attacks aimed against the Dyn Managed DNS infrastructure."

4:37 p.m. ET:

"Our engineers continue to investigate and mitigate several
attacks aimed against the Dyn Managed DNS infrastructure."

4:59 p.m. ET:

"Our engineers are continuing to investigate and mitigate several
attacks aimed against the Dyn Managed DNS infrastructure."

6:20 p.m. ET:

"The incident has been resolved."

CNBC reported that Amazon investigated the issue as well,
although it eventually found the issue to be with the Dyn attack.
"Amazon & DynDNS investigating internet outage reports on
east coast of U.S. amid reports of major websites not working
properly," it tweeted.

Earlier this month, the United States
transferred its oversight of DNS to an international
non-profit group, a move that had been more than 20 years in the
making.

Here's a map of reported outages as of 9:20 a.m. ET, via Down
Detector: